The present invention relates to temperature compensation and, more particularly, to a temperature compensation method for a turbidity sensor used in an appliance for washing articles.
Reducing the amount of energy consumption in appliances or machines for washing articles, such as dishwashers or clothes washers, is a significant problem, in part because a large amount of energy is needed to heat the liquid, such as water, for washing such articles. Thus, decreased liquid consumption for such machines may result in significant improvements in energy efficiency. Several techniques are available to indirectly monitor cleanliness of the articles, including a device for measuring or sensing the turbidity of the liquid used to wash the articles.
Turbidity sensors that employ an electromagnetic radiation source, such as a light emitting diode (LED) for emitting electromagnetic radiation which propagates within the cleansing liquid, typically suffer from temperature variation effects, such as power output variation as a function of temperature. The temperature variations effects, if left uncorrected, can substantially degrade the accuracy of the turbidity sensor. For example, an LED having a temperature coefficient of about 4,000 parts per million (ppm) per .degree. C. can result in unacceptable accuracy over the temperature range of operation of the turbidity sensor. Thus, it is desirable to provide a temperature compensation method which substantially reduces turbidity sensor errors due to temperature variation effects in the LED. It is further desirable to provide a temperature-independent intensity referencing scheme for obtaining normalized turbidity measurements which are free from temperature variation effects. This is desirable since known intensity referencing or normalization schemes, such as disclosed in U.S. Pat. No. 5,083,447 by M. Kiuchi, et al, typically rely on turbidity measurements taken at temperatures which can be very different from the actual temperature encountered during any cycle of operation of the washing machine and therefore the accuracy of such intensity referencing schemes is inherently limited by the temperature response of the sensor.